Air on Earth has a fairly constant composition
of 20% oxygen (O2 gas) and 80% nitrogen
(N2 gas) all over the world and at up
to several miles above sea level. This means that
for every 100 molecules in the air 20 would be
oxygen and 80 would be nitrogen. What does change
is the density of the air as you travel up from
sea level, the higher you go the fewer air
molecules, and so fewer oxygen molecules. This is
because as you go higher there is less air above
you to push down on you and squeeze the air
molecules together around you.

If you are
doing any special calculations for a project and
you have found a number in a book that tells you a
fact about oxygen at sea level you can use that
same number for 20 feet above sea level. Any
difference will be very tiny.

Answer 2:

The same amount through out the entire bottom
10 km of the atmosphere DUE TO MIXING BY
TURBULENCE... the composition of the atmosphere is
for all intents and purposes the same everywhere
on average.

Answer 3:

About 20% of the total atmosphere, essentially
the same as at sea level. In order to get to
having half the amount of oxygen at sea level, you
would need to get to about 20,000 feet.

Answer 4:

Oxygen makes up about 20.9% of the volume of
air at low elevations. It should not be much
different between sea level and 20 feet above sea
level, as gasses can move around freely.